The term "reproduction material" is used herein to refer to a sheet-like material which, upon impact or subjection to pressure, is capable of transferring pigment to an underlying copy layer. Such materials, in sheet form, have commonly been referred to as carbon paper and have been available in single-use or multiuse form. The single-use carbon paper is discarded after use while the multiuse carbon paper is capable of being subjected to numerous impressions, each of which is capable of transferring a portion of the pigment on the surface opposite the impact face to successive sheets. Such materials are also available as so-called carbon ribbons, i.e. strips of film provided with a pigmentatious layer and passed between the platen and the type faces of a typewriter or other printing device.
In recent years, such materials have relied more heavily upon synthetic resin films as pigment carriers and thus, to an extent, the term "paper" is a misnomer.
The multicarbon materials with which the present invention is concerned, therefore, are such print-transfer materials as utilize a foil as the pigmentatious-layer carrier and can be in the form of sheets, ribbons or the like. The pigmentatious layer is applied in multiuse or multicarbon materials to the carrier foil in the form of a color-release coating which comprises a synthetic resin matrix in which the pigmentatious agent is displaced.
This pigmentatious agent can be a coloring substance such as a dye or a pigment in an oil phase so that it is, in effect, a colored oil which is not capable of solubilizing and is not soluble in the synthetic resin matrix.
To produce this layer, a solution of the matrix synthetic resin in a solvent is formed, this solvent also constituting a solvent for the colored oil. The solution is applied to the carrier foil and upon evaporation of the solvent, the oil is found to be disposed in numerous practically mutually communicating microdroplets embedded in the synthetic resin matrix.
Upon impact, some of the oil is transferred to the copy sheet in the region of the impact to form a copy of the impression and colored oil can refill the pores from which the transferred oil has been driven out.
Such multicarbon materials thus have a spongy layer filled with the coloring agent and from which the coloring agent is transferred to the copy sheet by force of the typewriter character or other printing impression.
An important disadvantage of earlier multicarbon materials of this type is the relatively low yield of the material. In other words, while the material is intended to produce a number of copies, in practice even after a few uses the transferred impression can be seen to be faded by comparison with the actual impression. Repeated impacts at the same location rapidly deplete the pigmentatious material in this region so that the copy after only a few uses is poor. The transfer material must then be discarded.
The problem is even more pronounced with so-called carbon ribbons since strike-overs at a single location are even more common and it is possible that even after a single passage through the machine, some regions of the ribbon may have been overstruck several times. As a result, on the return pass of the ribbon, the typewritten result is of poor quality. Clearly repeated uses of the ribbon suffer this problem even more markedly and the problem is still further pronounced in rapidly operating automatic printers, computer-associated and word processing equipment.
Ribbon changing is often a time-consuming and complex operation which interferes with the continuous operation of the machine. The problem has been traced in part to the fact that the concentration of oil droplets in the synthetic resin matrix cannot be made as high as is desired because increasing concentrations are detrimental to the stability of the synthetic resin matrix and prevent it from satisfactorily withstanding impact.
As a practical matter, therefore, the amount of coloring matter which can be contained in a unit area of the color transfer layer is limited. Furthermore the desired degree of intercommunication between the individual droplets may not materialize because of this limitation in the concentration of the oil droplets so that depleted vacuoles or pores in the synthetic resin matrix may not be replenished from adjacent vacuoles or pores.
The prior art materials have other disadvantages which are also significant. For example, it is difficult to bond the pigmentatious layer sufficiently firmly and uniformly to the carrier foil. This is because the pigmentatious layer has its pores not only open toward the copy sheet or paper but also open toward the carrier force and as a result of impact the oil is pressed from these pores and tends to penetrate between the foil and the matrix layer. With increasing uses, therefore, the spread of the oil film between the pigmentatious layer and the foil reduces the bonding of the matrix layer to the foil. Increasing the use, folding, creasing, and other factors which may interfere with effective bonding can cause large portions of the pigmentatious layer, in flakes or larger sections, to ablate from the foil.
Because of this problem it has been proposed to provide intermediate layers between the pigmentatious layer and the foil to serve as bonding layers or adhesion-increasing layers.
The application of such adhesion-promoting coatings to the foil requires an added step in the production of the material and hence results in an increse of the cost thereof.
Furthermore, the use of such intermediate layers is not free from problems. For example, the intermediate layer must have a high elasticity and itself possess good adhesion to both the carrier foil and to the pigment coating. It also must have high internal cohesion. It also must be of a composition such that it is not attacked by the solvent during the application of the pigmentatious layer.
In addition, this intermediate layer must be of a composition such that it does not permit migration of the color-carrying oil out of the pigmentatious layer.
In practice, no universally acceptable intermediate layer has been developed heretofore and hence for each carrier foil, matrix material composition and pigmented oil experiments must be undertaken to develop satisfactory intermediate layers. Frequently these intermediate layers must be so expensive that their use in multicarbon materials for the purposes described is barred by practical consideration.